1. FIELD OF THE INVENTION
This invention relates to improvements for a multi-phase hybrid stepper motor.
2. DESCRIPTION OF THE PRIOR ART
Hybrid stepper motors are known to be excellent actuators whenever high precision positioning is required, for more than twenty-five years. Areas of application include any kind of handling-equipment in manufacturing, up to full process automation, as well as in the computer peripherals industry (i.e. printers, plotters, facsimiles, and disc-drives).
A. The Four-Phase Hybrid Stepper Motor
The most common motor is a so called four-phase motor with 8 mainpoles. FIG.9 (a) and (b) illustrate cross-sectional views A--A and B--B of FIG. 10, the four-phase hybird stepper motor. Each of these statorpoles has a winding 11 and 5 teeth 12. As illustrated in FIG.10, teeth 12 face to the inner rotating teeth 15 of rotorcaps 14 (hole cap, at least two) with are mounted one on each side of a an axial magnetized permanent magnet 13. In this case the rotorcaps 14 have 50 teeth 15 each, on creating north-poles, and the other one creating south-poles with a toothpitch of 360/50 degrees=7.2 degrees. Two rotorcaps 14 are offset to each other in rotational direction by one half toothpitch. These rotorcaps 14, mostly made out of silicon lamination material or solid magnetically soft iron, together with the magnet mounted on a shaft 16, represent a rotor system.
Currently there are hybrid stepper motors on the market with more than one rotor system. Most commonly, the toothpitch of the statorpole teeth is the same as the rotor toothpitch, Tp. In FIG.9 adjacent poles in this case are 45.degree. away from each other center of middle tooth of the adjacent pole). The physical shiftangle between any adjacent pole is now 45.degree. divided by 7.2.degree. (toothpitch) minus integer numbers of toothpitch, and equals 1/4 of the toothpitch. This is equivalent to 90.degree. electrical, since one toothpitch represents 360.degree. electrical.
This stator and its lamination can be considered to be a symmetrical stator-lamination, since every pole has the same number of teeth with the same toothpitch and the same winding-slot-opening between adjacent outer teeth of adjacent poles. In this case 2.25 Tp minus toothwidth.
Theoretically these motors could be built with 48 stator teeth (6 teeth per pole), if the slot openings in the case of 1.25 Tp minus one toothwidth allow the insertion of the winding and create a phase-inductance as required.
These motors can have a symmetrical lamination for any number of rotor teeth which fulfills the following requirement of:
Number of rotor teeth divided by number of stator mainpoles equal to K +1/4, K being an integer. As an example, 50/8=6.25 will create a symmetrical lamination. 48/8=6, does not work because there is no shiftangle. For instance, any motor with a number of rotor teeth NR=8(K+0.25) can have a symmetrical stator. So NR=10; 18; 26; 34; 42; 50; and 58 will allow a symmetrical lamination with 8 stator-poles.
These motors create a sinusoidal detent-torque which is periodical with on fourth of the toothpitch and keeps the rotor in position without any external statorpole excitation.
When considering that every pole when energized, together with the rotor system develops a nearly sinusoidal holding torque (FIG.11), which is periodical with the toothpitch Tp, and since each pole is 90.degree. electrical away from each other, one can design the following torque-vector-deagram, FIG.12. It is easy to see that phase I is given by poles 1 and 5, phase II by poles 2 and 6, phaseIII by poles 3 and 7,and phase IV by poles 4 and 8. It can also be seen that maximum torque is obtained when all of the 8 poles are energized. A fullstep angle of .alpha.F=90.degree. electrical or .alpha.F=1/4 Tp will be achieved when the polarization of four phases (i.e. 1; 5; 3; 7) gets changed at the same time. A halfstep angle of .alpha.H=45.degree. or 1/8 Tp will be reached when those aforementioned poles are switched off instead of being changed in polarization.
In this case the torque varies in halfstep mode by .sqroot.2 to 1. Since two poles are always 180.degree. away from each other (i.e.1 to 3 or 2 to 4), and also since the maximum torque will develop from step to step when 4 poles get changed in polarity at the same time, phase I and III can be connected with each other as well as phases H and IV . This allows a relatively simple, so called H-Bridge driver with a total of 8 transistors to achieve fullstep and halfstep modes.
When energized in these two modes, the 8 poles have a polarization as shown in FIG .19. Here step 1 represents a fullstep position and step 2 a halfstep position.
As it can be seen in FIG. 19, this motor has as many north-poles as south-poles at any time. This means that the radial forces on the north-rotorcap are the same as they are on the south-rotorcap. This is important in order to avoid major bearing damage over lifetime, as well as to insure a better reaction against mechanical inaccuracy to keep small stepangle tolerances.
From FIG. 13 it is also visible that whenever all 8 poles are energized, we have two north-poles adjacent, followed by two adjacent south-poles. This gives good stability at fullstep positions, since FIG. 11 shows the detent (fourth harmonic) will not support the stiffness of an 8-stiffness of an 8-pole energized holding horque when inphase with a single pole (FIG. 11 shows the detent offphase).
All of the discriptions above can be summarized by the following general laws for symmetrical hybrid stepper motors:
1. Number of rotor teeth given by EQU NR=mp[(n-1)+(K+1/m)] PA1 m=number of phases PA1 p=number of poles per phase PA1 K=positive integer equal or greater than one PA1 n=number of teeth per pole PA1 2. Highest possible number of stator teeth EQU NS max=NR-p PA1 3. The smallest possible number of stator poles is equal to the number of phases. This means that the smallest possible number of poles per phase is one. EQU .vertline.mp.vertline.min=m PA1 4. Stator toothpitch Tp can be the same as rotor toothpitch EQU TpS=TpR PA1 5. The basic detent-torque as well as the torque variation at speed (cogging) is periodical with the m.sup.th harmonic of the rotor toothpitch. PA1 6. Number of steps per revolution for: PA1 .alpha.F=360.degree. /mNR and PA1 .alpha.H=180.degree. /mNR PA1 1. Large variation in dynamic torque during switching from step to step and also in static positions when in halfstep mode. PA1 2. Relatively high detent-torque which can destroy the stepangle accuracy as well as the movement during micro-stepping. PA1 3. For micro-stepping mode a sinewave-current variation requires very small changes at the top of the sinewave. With small motors the rotor may not move because the torque change is too small. PA1 4. These motors develop relativery strong resonances at step frequencies below 1 kHz due to the torque variations. Sometimes those motors have speed zones in which the rotor will not rotate without a certain load or additional damper. PA1 5. Start-and stop-stop frequencies are relatively low. PA1 6. Due to the limited number of possible rotor teeth, most of the achievable step angles and the number of steps per revolution are not very practical for many industrial applications. PA1 1. Even though the detent-torque is small, it can reduce the stepangle accuracy, since it can change the holdingtorque equilibrium point.fwdarw.FIG. 16. PA1 2. When in halfstep mode with 5 or 3 phases on, the number of stator north-poles will never be the same as the number of stator south-poles.fwdarw.FIG. 20. This creates an imbalance in the radial force(.fwdarw.FIG. 18) on the two rotorcaps, which puts more force on the bearings. It is also more sensitive to manufacturing tolerances and reduces the stepangle accuracy. Since the number of north-and south-poles will change every second step inhalfstep mode, the radial force on the rotorcaps will change every second step. This, together with the 4(or 2) largerwinding-slot openings,creates larger vibrations at some speed frequencies. PA1 3. The difference in the number of north-and south-poles, and the variation at every second step, creates higher hysteresis-values in halfstep mode, since the tooth-and pole-induction will be higher over the rotorcap with the smaller number of north-or south-poles. PA1 4. A five-phase motor with a shiftangle between adjacent poles of 3/5 Tp, such as this, will not allow the same polarization on two two adjacent poles, as described for the motor with four phases. This reduces the stability in the equilibrium positions. As mentioned above, conventional four or five-phase stepper motor has many faults. The present invention is formed in order to solve the above mentioned faults, and it is an object of the present invention to provide a hybrid stepper motor improving in torque-stiffness with reducing the torque variations to a minimum, and in capable of reducing the resonant vibrations during rotation.
where:
and EQU NS=mpn
therefore Ns has to be always smaller than NR
with TpR=360.degree. /NR
But for any symmetrical motor lamination the stator tooth can also be Tps =360.degree. /NR-p
fullstep mode: NREVF=mNR PA2 halfstep mode: NREVH=2mNR
Consequently the stepangles are
B. Disadvantages of Hybrid-Stepper Motors with Four (or less) Phases
The variation factor is 1/.sqroot.2.
C. The Five-Phase Hybrid Stepper Motor Some of the above mentioned disadvantages can be eliminated when going to a higher number of phases.
The U.S. Pat. Nos. 3,866,104 and 4,000,452 describe a five-phase hybrid motor which follows the same basic laws (equations) as layed out above.
But in this case, since the number of phases divided by two (2) is not creating an integer and since in a PM-motor a change in current-derection is shifting the polariry by 180.degree. electrical (FIG.13), which is two times higher than normal.
Therefore: EQU NREVF=2mN.sub.R and .alpha.F=180.degree./mNR EQU NREVF=4mN.sub.R and .alpha.H=90.degree./mNR
The basic U.S. Pat. No. 3,866,104 describes a five-phase motor with a shiftangle between adjacent poles of 3/5 Tp. Therefore an electrical angle of 72.degree. (1/5 Tp) will be achieved between poles 2 and 4 (FIG.14).
In this case we will achieve a torque-vector-vector-diagram as shown in FIG.15, and a pole polarization table as shown in FIG. 20.
These two figures show that one can achieve fullstep with 5 phases on as well as with 4 phases on. Halfstep will be achieved when alternating between 5 phases on and 4 phases on. The torque difference in halfstep mode is just 5(percent). The cogging-torque as well as the detent-torque are the 5th harmonics (FIGS.16). This is a great improvement compared to the earlier described motor.
The resonance-frequencies are in a much higher range and no "none-running" speed zone can be realized.
The possible number of rotor teeth for symmetrical laminations follows the equation EQU NR=5p[(n-1)+(K+3/5)]
Therefore one can achieve rotor teeth numbers like 16; 26; 36; 46; 56; etc. Only the number 36 is of industrial interest or importance. Numbers like 50 or 100, which are very important for many applications, cannot be obtained in a symmetrical design as such. As U.S. Pat. No. 4,095,161 describes however, a symmetrical five-phase lamination with a shiftangle of 3/5 Tp can be used together with 4 different numbers of rotor teeth. As an example, the lamination shown in FIG. 14 is designed for NR=36, but it can be used also with NR=32,38,42. Numbers such as NR=20,30,40,50, etc. are only possible if we go to a nonsymmetrical lamination design, as described in U.S. Pat. No. 3,866,104. This means that the maximum number of stator teeth will be less since there are always 4 rotor toothpitches(T.sub.p), which cannot be used in conjunction with stator teeth.
A lamination design of such a motor is shown in FIG. 17(NR=50). The basic equation for such a five-phase nonsymmetrical motor is: NR=5p[(n-1)+(K+3/5)]+4 The smallest possible difference between NR and NS is 5p. Therefore for a 10pole motor the smallest difference is 10, which means a motor for 500 steps per revolution, with an angle of .alpha..sub.F =0.72.degree. , NR will be 50, and NS will be 40. This results in a decrease of the maximum possible torque and has to be considered as a disadvantage. Other disadvantages are the following items: